This invention relates to coated ceramic particles having a coat-forming substance applied to the surfaces of ceramic particles. The invention also relates to a process for producing ceramic-base sinters by sintering those coated ceramic particles or mixtures containing those particles, as well as ceramic-base sinters produced by the process.
To enhance the performance of ceramic-base sinters, providing an enhanced composite microstructure for the ceramic-base sinters is insufficient and it is also necessary to control the microscopic regions of that structure by suitable techniques such as refining and homogenization. To this end, feed powders have customarily been used that comprise fine ceramic particles with an average diameter of no more than 10 .mu.m and it is essential to prepare feed powders that comply with the above-mentioned objective at the stage of their preparation.
Conventionally, such feed powders have been prepared by ball milling, vibration milling, and other powder mixing methods. However, the incorporation of impurities during mixing is unavoidable in these methods; what is more, there is theoretically a limit on the extent to which a uniform structure can be realized. Even if the particles of a sintering aid and various other additive substances in powder form are fine, it is very difficult to insure ideal uniform mixing or uniform dispersion in such a way that the powder particles of additive substances will cover all of the ceramic particles present. Even if such uniform dispersion is realized, the extent of "uniformity" that can be attained is limited since the powder particles of additive substances are mixed on a particle basis. Particularly in the case where those particles are used in relatively small amounts, an uneven distribution will occur inevitably.
In many actual cases, the ceramic particles and the powder particles of additive substances agglomerate to form lumps in ceramic-base sinters or they occur unevenly in the sinter, eventually leading to marked deterioration in the performance of the ceramic-base material.
Therefore, in order to realize homogenization, it is necessary that the above-mentioned additive substances which will eventually be incorporated into the ceramic-base material should be distributed positively to all individual ceramic particles. To this end, the production of coated ceramic particles in which additive substances have been applied uniformly to the individual ceramic particles by coating techniques, as well as the production of high-performance ceramic-base sinters by sintering those coated ceramic particles are strongly desired.
With materials having definite uses, deliberate use of relatively large ceramic particles such as those exceeding 10 .mu.m in average diameter is very effective.
Consider, for example, hard and wear-resistant ceramic sinters; they can be produced by sintering grain-dispersed ceramic particles which are characterized by a dispersion of superhard ceramic particles in comparatively large amounts and, in this case, the superhard ceramic particles used as the feed material are important. On the other hand, ceramic-base sinters of high tenacity can be produced by sintering ceramic particles a comparatively large portion of which is comprised of non-regular particles such as plates and rods that have somewhat large aspect ratios; in this case, too, ceramic particles having more or less high aspect ratios are very useful.
Ceramic-base sinters which have these ceramic particles dispersed therein are characterized in that the effect of particle dispersing is remarkable since the ceramic particles are sintered as they are closely bound to the surrounding microstructure without imperfections or pores. To meet the end, however, the presence of sintering aids or binders that promote the sintering of those ceramic particles together with the surrounding microstructure, or additives such as grain boundary controlling substances which will help retain the grain morphology of the ceramic particles is essential.
Conventionally, the addition of additive substances such as sintering aids and binders or grain boundary controlling substances has been effected by powder mixing methods as in the case of fine ceramic particles with an average diameter of no more than 10 .mu.m.
However, as described above, the incorporation of impurities during mixing is unavoidable in this method; what is more, there is theoretically a limit on the extent to which a uniform structure can be realized. Even if the particles of the additive substances are extremely fine, it is very difficult to insure ideal uniform mixing or uniform dispersion in such a way that the particles of the additive sbstance powder cover all of the ceramic particles present. Even if such uniform dispersion is realized, the extent of "uniformity" that can be attained is limited since the particles of the additive substance powder are mixed on a particle basis. Particularly in the case where those particles are used in relatively small amounts, an uneven distribution will occur inevitably.
In many actual cases, the dispersed ceramic particles lump together or the particles of the additive substance powder agglomerate, as a result, the ceramic particles will form lumps in the ceramic-base sinter or they occur unevenly in the ceramic sinter, eventually leading to marked deterioration in the performance of the ceramic-base sinters.
Therefore, it is necessary that the additive substances be distributed positively to all individual ceramic particles. Further, in order to assure that the ceramic particles are sintered as they are closely bound to the surrounding microstructure, it is required to provide a highly controlled uniform coating on the surfaces of the ceramic particles; the coating is uniform in the surfaces of the individual ceramic particles and it is highly controlled in that this uniform coating is applied to all individual ceramic particles in the same manner. What is more, the greater the size of these particles, the more uniform this highly controlled coating should be in order to further reduce the uncovered portions of the particles.
Thus, the production of coated ceramic particles covered with uniform coatings that are highly controlled in this manner, as well as the manufacture of high-performance ceramic-base sinters using those coated ceramic particles are strongly desired.
Irrespective of their diameter, ceramic particles are conventionally provided with coatings by various techniques such as vapor-phase processes and wet plating methods. The vapor-phase approach has major features that are unattainable by other coating techniques, such as: (1) easy control of the atmosphere; (2) the selection of coat forming substances is basically unlimited and various kinds of substances including elemental metallic substances (e.g. active metals), nitrides, carbides, borides and oxides can be applied; (3) the desired coat forming substance can be applied without letting impurities be incorporated; and (4) the coating weight of the coat forming substance can be controlled freely.
However, if the ceramic particles are fine grains, it has been impossible to coat them individually by the vapor-phase methods for the following reasons.
First, in the case of a core particle powder composed of fine ceramic particles, the individual ceramic particles are cohesive enough to have a great tendency to agglomerate together, whereby almost all single particles form agglomerates. Since these agglomerates cannot be disintegrated unless they are subjected to a special action greater than their cohesive force, they cannot be simply coated as such to insure that the surfaces of the individual particles are covered with the coatings of said coat-forming substances, eventually yielding coated agglomerates in which the surfaces of the agglomerates are covered with the coatings of the coat forming substances.
This has caused a problem with the individual agglomerate-forming particles in that the surfaces of the particles located on the surfaces of the agglomerates have large coating weights but suffer from uneven coating whereas the particles located within the agglomerates are not covered at all.
In the case of ceramic particles exceeding 10 .mu.m in average diameter, it has also been impossible to form the above-described highly controlled uniform coatings by the various coating apparatus and methods heretofore proposed as known techniques and the reasons are as follows.
With ceramic particles having an average diameter in excess of 10 .mu.m, the cohesive force is not as strong as what develops in fine grains having an average diameter of 10 .mu.m or less and, yet, it has been impossible to insure that the core ceramic particles in powder form will become discrete to occur on a single-particle basis. Hence, those portions of agglomerates which are blocked by other ceramic particles remain uncoated on the surfaces. As already mentioned, highly controlled uniform coatings are needed and, yet, even the small cohesive force discussed above has been so much influential as to cause a very serious problem in actual cases.
With a view to solving these problems irrespective of the diameter of ceramic particles to be coated, attempts have already been made to coat the particles in a dispersed state in order to assure the coating of the surfaces of the individual particles in the powder of core particles.
For instance, Unexamined Published Japanese Patent Application (kokai) Sho 58-31076 teaches an apparatus and method, according to which a vessel placed in PVD equipment is charged with the particles in a powder of core particles and vibrated by an electromagnetic means so that the core particles in the vessel are rolled as they are coated by a PVD process. Unexamined Published Japanese Patent Application (kokai) Sho 61-30663 teaches an apparatus, according to which a vessel placed in PVD equipment is charged with the particles in a powder of core particles and vibrated by a mechanical means so that the core particles in the vessel are rolled as they are coated by a PVD process. However, in the actual practice with those apparatus or methods in which the vessel is vibrated so that the particles in the powder of core particles which are ceramic particles are rolled as they are provided with coatings, the necessary action for disintegrating the agglomerates of ceramic particles with an average diameter not greater than 10 .mu.m by applying a force exceeding their cohesive force cannot be produced and, hence, the agglomerates cannot be disintegrated; to the contrary, a granulating action develops to form agglomerates that are greater in number or size than before the powder of core particles is supplied into the vessel. On the other hand, ceramic particles having an average diameter in excess of 10 .mu.m are simply subjected to a sliding action as they form many layers in superposition and it has been impossible to achieve the desired coating of single separate particles.
Unexamined Published Japanese Patent Application (kokai) Hei 3-153864 teaches an apparatus and method, according to which a rotating vessel having barriers and/or ridges and grooves in the inner surface is charged with core particles and rotated as the surfaces of the particles are coated by an evaporation method. The problem with this apparatus and method is that the necessary action for disintegrating the agglomerates of ceramic particles with an average diameter not greater than 10 .mu.m by applying a force exceeding their cohesive force cannot be produced and, hence, the agglomerates cannot be disintegrated and, what is more, an increased number or size of agglomerates will simply form. On the other hand, ceramic particles having an average diameter in excess of 10 .mu.m are simply subjected to a gentle stirring action as many of them contact one another forming many layers in superposition and it has been impossible to achieve the desired coating of single separate particles.
Unexamined Published Japanese Patent Application (kokai) Sho 58-141375 teaches an apparatus in which the particles of a powder in a reactive gas atmosphere are suspended by the flow of the reactive gas under gravity and in which the surfaces of the particles are coated with the precipitating substance that forms by the chemical reaction involving the reactive gas. Unexamined Published Japanese Patent Application (kokai) Hei 2-43377 teaches a method in which particles placed under vacuum are fluidized as they are subjected to coating by a thermochemical reaction treatment. Unexamined Published Japanese Patent Application (kokai) Sho 64-80437 teaches a method in which the agglomerates of core particles in powder are disintegrated by a sound wave that is a composite of low and high frequency waves, so that the agglomerates are fluidized to improve the coating efficiency. However, these techniques which utilize the fluidized bed of the particles in a powder of core particles which is formed by a gas flow or vibrations have had the problem that with ceramic particles having an average diameter of no more than 10 .mu.m, it is practically impossible to fluidize the separate individual ceramic particles, thus failing to disintegrate the agglomerates of these ceramic particles. On the other hand, with ceramic particles exceeding 10 .mu.m in average diameter, it is practically impossible to insure that all of these ceramic particles are similarly and independently fluidized and suspended as single separate entities and one has been incapable of eliminating uneven coating of the particles which is due to the hiding of one ceramic particle by another.
Unexamined Published Japanese Patent Application (kokai) Sho 54-153789 teaches an apparatus in which a powder material is dropped within a vacuum vessel, where the metal vapor is generated to form a metal coating on the particles. Unexamined Published Japanese Patent Application (kokai) Sho 60-47004 teaches a method in which a monomer gas and the particles of a powder are introduced into a high-frequency plasma region in a vacuum vessel, where a coating film of an organic substance is formed by plasma-assisted polymerization. If ceramic particles with an average diameter of no more than 10 .mu.m are simply introduced as in the techniques described above, agglomerates of the ceramic particles cannot be disintegrated. On the other hand, ceramic particles exceeding 10 .mu.m in average diameter will simply drop while forming agglomerates which are not single separate particles and various problems occur, such as uneven coating due to the hiding of one particle by another, the total failure of the particles within an agglomerate to be coated, and differences in the coating weights of individual particles.
Unexamined Published Japanese Patent Application (kokai) Sho 64-80437 teaches a method in which the agglomerates of core particles in powder are disintegrated by a sound wave consisting of low and high frequency waves so that they are fluidized to improve the coating efficiency. However, this method which imparts vibrations to the fluidized bed has had the problem that with ceramic particles having an average diameter of no more than 10 .mu.m, it is practically impossible to fluidize the separate individual particles, thus failing to disintegrate the agglomerates of these particles. On the other hand, with ceramic particles exceeding 10 .mu.m in average diameter, it is practically impossible to insure that all of these particles are similarly and independently fluidized and suspended as single separate entities and one has been incapable of eliminating uneven coating of the particles which is due to the hiding of one particle by another.
Unexamined Published Japanese Patent Application (kokai) Sho 62-250172 teaches an apparatus and method, according to which a powder that has been preliminarily treated by jet milling is allowed to stay within a chamber for heat treatment under vacuum, where it is subjected to a heat treatment and thence dropped under gravity through a powder feeder into a cylinderal sputtering chamber equipped with a vertical target, whereby the particles in powder are provided with a coating. Unexamined Published Japanese Patent Application (kokai) Hei 2-153068 teaches an apparatus and method, according to which a powder that has been preliminarily treated by jet milling is allowed to stay within a chamber for heat treatment under vacuum, where it is subjected to a heat treatment and thence introduced through a powder feeder into a rotary vessel accommodating a sputter source within a sputtering chamber in the form of a powder (not as single particles), with sputtering being effected as the vessel is rotated. These techniques involve a heating step which is performed before coating so that the jet-milled powder of core particles of ceramic particels is allowed to stay for heat treatment and because of this staying of the powder in the heating step, the ceramic particles of any diameter will form agglomerates again which are not single particles and, eventually, such agglomerates will not revert to single particles in the coating step.
Thus, none of the so far proposed techniques have successfully solved the problems associated with the apparatus or method for providing coatings on the core particles in powder which are ceramic particles. In actual cases, ceramic particles having an average diameter of not more than 10 .mu.m form agglomerates which cannot be disintegrated and, hence, no methods or apparatus have been available for producing coated ceramic particles in which said ceramic particles, being dispersed as single particles, are covered on their surfaces with coat forming substances.
Speaking of ceramic particles exceeding 10 .mu.m in average diameter, these particles are in actual cases subjected to a coating treatment in the form of agglomerates in which they remain in mutual contact and, hence, those portions which are blocked by other particles remain uncoated. As already Speaking of ceramic particles exceeding 10 .mu.m in average diameter, these particles are in actual cases subjected to a coating treatment in the form of agglomerates in which they remain in mutual contact and, hence, they have not ever been given highly controlled uniform coatings. In other words, no methods have been available for producing ceramic particles with highly controlled uniform coatings, nor have been apparatus for implementing such methods. Hence, irrespective of the diameter of staring ceramic particles, it has been impossible to prepare coated ceramic particles by providing each of those starting particles with a controlled uniform coat by vapor-phase coating techniques using bonding material forming substances and/or sintering aid forming substances and, it has accordingly been impossible to produce the aforementioned ceramic-base sinters of high performance.